Hydraulic metal powder press

ABSTRACT

A metal powder press having frame means, lower ram means supported on the frame means for vertical movement. A hollow tool matrix is mounted on the lower ram means and movable vertically therewith. Lower press means is receivable in the lower end of the hollow tool matrix. Upper press means are receivable in the upper end of the hollow tool matrix. Metal powder storage means are provided to supply powder to a filling shoe for selectively filling the hollow tool matrix with metal powder.

United States Patent 1191 Silbereisen et al.

1451 Jan. 29, 1974 HYDRAULIC METAL POWDER PRESS Inventors: Hermann Sllbereisen; Felix Wlnterberg, both of Lochnerallee; Severin Ginzel, Muchen-Moosach, all of Germany Filed: Nov. 26,1971

Appl. No.: 202,117

Related US. Application Data Continuation-impart of Ser. No. 856,434, Sept. 9, l969, abandoned.

Foreign Application Priority Data Sept. 9, 1968 Germany 6751221 us. c1 425/78, 425/219, 425/260,

425/352 Int. Cl B22r 3/00 Field 61 Search 425/78, 219, 260, 352

References Cited UNITED STATES PATENTS 2/1970 Haller 425/78 2,550,653 4/1951 Harrington 425/78 2,859,502 ll/l958 Brown 425/260 x 2,592,768 4/1952 Van Der Pyl 425/260 x 3,132,379 5/1964 Crane 425/78 994,349 6/191 1 Updegraff 425/96 Primary Examinerl-l. A. Kilby, Jr. Attorney, Agent, or Firm-Robert E. Woodhams et al.

ABSTRACT A metal powder press having frame means, lower ram means supported on the frame means for vertical movement. A hollow tool matrix is mounted on the lower ram means and movable vertically therewith. Lower press means is receivable in the lower end of the hollow tool matrix. Upper press means are receivable in the upper end of the hollow tool matrix. Metal powder storage means are provided to supply powder to a filling shoe for selectively filling the hollow tool matrix with metal powder.

14 Claims, 6 Drawing Figures HYDRAULIC METAL POWDER PRESS This application is a continuation-in-part application of Ser. No. 856,434, filed Sept. 9, 1969 now abandoned.

BACKGROUND OF THE INVENTION The present invention deals in general with hydraulic metal powder presses in which fine metal powder is compressed to a workpiece in a mold cavity under high pressure by hydraulically operated press members, the shape of which workpiece corresponds to the mold cavity. The height of the workpiece depends on the amount of metal powder introduced into the mold cavity before the compression operation. The compactness or density of the workpiece depends on the magnitude of the pressure which is transferred from the hydraulically operated press members onto the powder. In order to be able to effectively transfer this pressure onto the metal powder, it is necessary that the hydraulically operated press members are aligned exactly with the mold cavity especially in the case where high forces occur during operation. Further, inorder to be able to control the amount of the metal powder which is filled into the mold cavity, the size of the mold cavity chamber must be able to be determined exactly.

It is an object of the present invention to provide an improved metal powder press in which the mold cavity is constructed in a matrix which is movable by hydraulic means relative to a die which penetrates from below into the mold cavity.

A further object of the invention is to provide a metal powder press in which a hydraulically operated upper ram penetrates for the purpose of compressing the metal powder from above into the mold cavity, whereby the compression path of the upper press member can be determined exactly by adjustable means.

A further object of the invention is to provide a hydraulic metal powder press with an indicating and adjusting device through which the path of stroke of the tool matrix can be exactly predetermined and adjusted relative to the lower press member.

Further objects and purposes of the invention will result from an inspection of the following description with reference being madeto the drawings described hereinbelow illustrating a preferred embodiment of the invention. 1

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic front view of an inventive metal powder press;

FIG. 2 is a schematic side view of the metal powder press illustrated in FIG. 1, viewed in the direction of the arrow A of FIG. 1;

FIG. 3 is an enlarged front view of a stop device for limiting the path of the upper ram of the metal powder press illustrated in FIG. 1;

FIG. 4 is an enlarged view of a detail of the metal powder press illustrated in FIG. 2 whereby the tool matrix is in a fill position relative to the lower ram, in which fill position the mold cavity of the tool matrix is supplied with metal powder by a fill device;

FIG. 5 is an illustration similar to FIG. 4 in which, after the compression process has taken place, the tool matrix is in a withdrawal position in which the compressed workpiece is discharged; and

FIG. 6 illustrates in an enlarged scale an indicating and adjusting mechanism of the metal powder press illustrated in FIG. 1, whereby the remaining parts of the metal powder press have not been illustrated.

DETAILED DESCRIPTION The metal powder press illustrated in FIGS. I and 2 comprises substantially a press frame 1 which has mostly a rectangular shape and in which an upper hydraulic cylinder asembly 2 and a lower hydraulic cylinder assembly 3 are installed. An upper ram 4 of the cylinder assembly 2 has a crosshead 5 secured at its free end. The upper ram is slidingly movable in the upper cylinder 2 and can be moved by supplying a hydraulic pressure medium to the cylinder through hydraulic lines which are not illustrated. An upper press member 6 is secured to the crosshead 5. The function of the press member 6 will be described in more detail hereinbelow.

The lower cylinder assembly 3 has a base plate 7 which is fixedly connected to the press frame 1. A plurality of horizontally spaced posts 8 are mounted on the base plate 7 and extend upwardly therefrom to support a support plate 9 for a lower press member 11.

The lower cylinder assembly 3 can, similar to the upper cylinder assembly 2, be charged with a hydraulic pressure medium through lines which are not illustrated to guide a slidingly movable lower ram shaft 10. A crosshead 10A is secured to the ram shaft 10. A plurality of horizontally spaced posts 12 are secured to the crosshead 10A and extend upwardly away therefrom. A tool matrix 13 is mounted on the support posts 12. The posts 12 extend through suitable guide openings 9A in the support plate 9. A mold cavity 14 is provided in the tool matrix 13 (FIG. 4) and comprises an opening 13A extending through the tool matrix 13. The cross-sectional shape of the upper press member 6 and the lower press member 11 corresponds to the crosssectional shape of the mold cavity. The mold cavity is axially aligned with the upper press member 6 and the lower press member 11.

The characteristics of the metal powder press described hereinabove are known in the prior art and do not form a part of the invention.

As can be seen from the side view of FIG. 2, a fill device for metal powder is arranged on the back side of the press frame 1. The fill device consists of a metal powder container 15 which is secured on the upper end of the press frame 1 by a bracket 15A. A hose 15B is connected to the funnel-shaped lower end of the container l5 and extends downwardly to a filling shoe 16. The filling shoe 16 is supported for movement on the upper surface 16A of the tool matrix 13 between a fill position (FIG. 4) and a retracted position (FIG. 5). A pressure-operated piston-cylinder arrangement 17 is provided for driving the filling shoe 16 for the aforesaid movement. The piston-cylinder arrangement 17 is secured on a platform 18 supported solely by a' post 19 which is screwed into a socket 20 which is secured on the outer end of a holding arm 21 fixed directly to the ram shaft 10. The post 19 can be rotated by a hand wheel 22 fixedly secured on the post 19. Thus, the plane of the platform 18 can be adjusted so that it is flush and coplanar with the upper surface 16A of the tool matrix 13.

A stop device 23 for limiting the movement of the upper ram 4 and the upper press member 6 is arranged on the press frame 1. The stop device 23 is illustrated in broken lines in FIG. 1. It is shown in an enlarged scale in FIG. 3 and consists substantially of support posts 24 which are arranged on opposite sides of the tool matrix 13 and which are supported adjacent the base plate 7 on the lower cylinder arrangement 3 on the press frame 1. The support posts 24 each consist of a threaded spindle 25 and a tube 26 having an internal thread. The tube 26 is screwed onto the upper end of the threaded spindle 25. Pressure plates 27 are releasably secured on the upper end of each tube 26, which pressure plates 27 cooperate with the outer ends of the crosshead 6 and function as a stop. The support posts 24 can be adjusted with respect to their height by rotat ing the threaded spindle 25. A shaft 28 is provided for this purpose and is supported on the press frame 1. Two spaced worms 29 are secured to the shaft 28 and engage a not illustrated worm gear secured as by a key to the lower end of the threaded spindle 25. The threaded spindles 25 are guided and held in vertical support bearings 30. If the shaft 28 is rotated by a hand crank 31 secured thereto, the threaded spindles 25 are rotated through the worm 29 and the not illustrated worm gears so that the tubes 26 move up or down depending on the direction of rotation. In order to prevent the tubes 26 from rotating with the threaded spindles 25, sleeves 33 are fixedly mounted with respect to rotation on the lower tubes 32 which surround the threaded spindles 25. The sleeves 33 cooperate with engagement surfaces on the tubes 26, such as by a not illustrated tongue and groove arrangement to prevent a rotation of the tubes 26. In place of this construction, however, any desired construction can be chosen which prevents a rotation of the tubes 26 with the threaded spindles 25.

FIG. 6 illustrates an indicating and adjusting mechanism with which the amount of the metal powder which is supposed to be filled into the mold cavity 14 (FIG. 4) of the tool matrix 13 is determined and which indicates the stroke of the lower ram shaft and the tool matrix 13 having the mold cavity 14 therein.

The indicating and adjusting device is arranged on the side of the hydraulic press, here the right side as illustrated in FIG. 1. The indicating and adjusting device includes two scales 35 and 36 which are arranged on the press frame 1. The scale '35 is fixedly secured on the press frame 1 while the scale 36 is mounted on a traveling nut 37 which can be adjusted lengthwise of a threaded spindle 38 in amanner which will yet be discussed more in detail. The traveling nut 37- is constructed at its left end in form of an indicator which upon a movement of the trave ling nut 37 lengthwise of the threaded spindle 38 is displayed in front of the stationary scale 35. An indicator 40 which is constructed as a polygon cooperates with both scales 35 and 36 and is movable lengthwise along the scales-on a further threaded spindle 39 by means of a traveling nut which is not illustrated secured to the polygon.

The threaded spindles 38 and 39 are rotatably supported in the press frame 1 in a manner which is only schematically indicated and can be rotated by means of hand wheels 41 and 42 (FIG. 1) through bevel gearing not illustrated in the schematic of FIG. 5.

A nonrotatable threaded spindle 43 is supported for movement in direction of its longitudinal axis beside the lower ram shaft 10. The support for the spindle 43 is indicated schematically at 43A. Two stop nuts 44 and 45 are spaced from one another and are adjustably mounted on the threaded spindle 43. The stop nuts 44 and 45 can be rotated by drive chains 46 and 47 and by sprocket wheels 48 and 49 which are keyed onto the threaded spindles 39 or 38, respectively in an axially aligned position to the stop nuts 44 and 45. An arm 50 which extends radially away from the lower ram shaft 10 but which is fixedly connected to it cooperates with the stop nuts 44 and 45. The threaded spindle 43 which can be moved in axial direction acts mechanically onto the slide valve of a check-valve 51 which is only schematically indicated in FIG. 6. The check-valve 5.1 interrupts, upon operation of the slide valve by movement of the threaded spindle 43, the oil feed to the cylinder 3.

OPERATION described hereinafter. Thereafter, by rotating the hand wheel 42, the indicator 40 is adjusted to a scale value of the scale 35 which corresponds to the uppermost position of the tool matrix 13. Since the indicator 40 gives at the same time a scale value on the scale 36, the difference between the uppermost and the lowermost position of the tool matrix 13'can be read directly on said scale 36. The check-valve 51 is not opened, for example-by hand by the operator, into a direction which permits the adding of hydraulic medium into the cylinder chamber of the cylinder 3, which chamber lies below the lower ram. Through this the lower ram shaft 10, the tool matrix 13 and the fill device consisting of the parts 16 to 22 are moved upwardly until the arm 50 abuts the stop nut 44. Through this the threaded spindle 43 is moved, and consequently the slide vlave of the checkvalve 51 which slide valve is connected to the threaded spindle 43, is operated. This results in an interruption of the supply of the pressure medium to the cylinder'3.

Through the movement of the tool matrix 13, the stationary lower press member 11 has moved out of the mold cavity 14, into which it earlier extended, and has through this cleared the mold cavity 14. The pistoncylinder arrangement 17 is now operated manually or automatically in the manner that the filling shoe 16 slides from the platform 18 onto the surface 16A of the tool matrix 13 until it covers the mold cavity 14 (FIG.

4). Since the filling shoe 16 is open on the bottom, the metal powder falls now from its inner chamber into the mold cavity 14 until same is filled. The filling shoe 16 is thereafter pulled back again by the piston-cylinder arrangement 17 so that the mold cavity 14 flush with the upper surface of the tool matrix 13 is filled with metal powder. A suitable seal at the lower edge of the filling shoe 16 takes care that the escape of metal powder during the moving'process is substantially preis exactly predetermined with reference to the upper side of the lower press member 11 and which corresponds to the height of the workpiece to be manufactured. During the downward movement of the upper press member 6, same moves from above into the open opening of the mold cavity 14 in the tool matrix 13 and compresses the metal powder provided therein until its movement is stopped by an abutment of the crosshead 5 against the upper side of the pressure plates 27. In this position of the upper press member 6, the work piece has received its desired height and shape.

Now again, either manually or automatically, the slide valve of the check-valve 51 is operated into such a direction that the feeding of pressure medium to the upper side of the lower ram in the lower cylinder 3 takes place. Through this the lower ram shaft 10, the tool matrix l3 and the fill device which consists of the parts 16 to 22 move downwardly relative to the lower press member 11. Through this movement the lower press member 11 enters again the mold cavity 14 and moves thereby the compressed solid workpiece out from the mold cavity (FIG. 5). The movement of the lower ram shaft lasts until the arm 50 abuts the stop nut 45 and moves thereby the threaded spindle 43 downwardly. Through this the slide valve of the checkvalve 51 is again moved into the blocking position so that the pressure medium supply to the cylinder 3 is interrupted and the lower ram shaft 10 is stopped. In this position again the mold cavity 14 is filled by the lower press member 11 so that the upper side of the lower press member 11 is coplanar with the upper surface 16A of the tool matrix 13. Now the finished workpiece can be removed and the process of manufacture is repeated.

The advantages of the inventive metal powder press are the following:

Due to the fact that the fill device which consists of i the parts 16 to 22 is secured not on the tool matrix but solely on the lower ram shaft 10, the loads and vibrations which are caused by the fill device effect to a smaller degree the position of the tool matrix 13 relative to the upper press member 6. Through this loading or stressing of the upper press member 6 in the mold cavity 14, which are possible in known machines, are reduced to a minimum.

By using mechanical stopsfor limiting the path of the upper press member 6 in form of the support posts 24, one obtains at all times a constant height of the manufactured workpieces. In the known machines, the braking or stopping of the upper press member is accomplished by interrupting the pressure medium supply to the upper cylinder. However, this interruption is independent from the reaction time of the switches and magnetic valves required herefor and from the viscosity of the pressure medium itself. Dependingon whether the pressure medium is warm or cold, one obtains, therefore, different reaction times and thus uneven workpiece heights.

By providing adjustable stops through which the movement of the lower ram shaft is limited and which are coupled mechanically with an indicating scale, the operator has the possibility to control by visual inspection the effective stroke of the tool matrix which is a measure for filling the mold cavity chamber 14.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. A metal powder press, comprising:

frame means;

lower ram means supported on said frame means for vertical movement relative thereto;

a tool matrix mounted on said lower ram means and movable vertically therewith, said tool matrix having means defining an opening therethrough the axis of which is vertically aligned;

lower press means receivable in the lower end of said opening in said tool matrix;

upper ram means having upper press means thereon adapted to be moved into and out of the upper end of said tool matrix;

metal powder storage means; and

filling means connected to said metal powder storage means for selectively filling said opening in said tool matrix with a metal powder, said filling means comprising a platform and mounting means for mounting said platform in said lower ram means and being free of a direct attachment to said tool matrix but movable vertically with said tool matrix, a fill shoe and support means for supporting said fill shoe for sliding movement into and out of communication with said opening in said tool matrix and power drive means for. moving said fill shoe between said in and out positions of communications with said opening.

2. A metal powder press according to claim I,

wherein said filling means further includes means for vertically adjusting said platform relative to the upper surface of said tool matrix so that the plane of said platform can be adjusted to be flush and coplanar with said upper surface of said tool matrix.

3. A metal powder press according to claim 2, wherein said adjusting means comprises a threaded support sleeve secured to said lower ram means and a threaded support column engaged with said threaded support sleeve and secured to said platform for supporting same, a hand wheel secured to said support column whereby a rotation of said hand wheel will rotate said support column to cause said threaded interconnection to effect an adjustment of said platform.

4. A metal powder press according to claim 1, including a two part scale means, one part being connected in association with said frame means and the other part being connected in association with'said tool matrix and including means for adjusting the relative positions therebetween.

5. A metal powder press according to claim 1, including means for limiting the downward movement of said upper ram means.

6. A metal powder press, comprising:

frame means;

lower ram means supported on said frame means for vertical movement;

a hollow tool matrix supported on said lower ram means and movable vertically therewith; lower press means receivable in the lower end of said hollow tool matrix;

upper ram means having upper press means thereon adapted to be moved into and out of the upper end of said tool matrix; and

at least two adjustable stop means for limiting the downward movement of said upper ram means relative to said tool matrix, said stop means being in the form of supporting columns which are concealed within the press frame and finely adjustable in height by means of a common shaft with the aid of worm gears and having removable pressure plates at their upper ends.

7. A metal powder press according to claim 6, wherein the supporting columns comprise a threaded portion which is driven by a worm gear, and a sleeve portion seated on the threaded portion, and the screw thread is covered with a protective sleeve.

8. In a hydraulic metal powder press having a frame, a matrix having a mold cavity therein, which matrix is movable in the frame in vertical direction by a pressuremedium supplied lower ram shaft connected to same, an upper ram having an upper press member which can be moved from above into the mold cavity of the matrix, which upper ram is vertically movable in the frame and is supplied with pressure medium, a lower press member which extends from below into the mold cavity, a fill device for filling the mold cavity with metal powder, which fill device can be moved together with the matrix in vertical direction and which has a filling shoe which can be moved slidingly on the upper surface of the matrix and on a platform which lies in the same plane as the upper surface of the matrix, which filling shoe is connected to a storage reservoir for metal powder and includes a piston-cylinder arrangement for moving the filling shoe back and forth, comprising the improvement wherein said platform carrying the fill device is supported solely on said lower ram shaft and being free of any direct attachment to said matrix.

9. Hydraulic metal powder press according to claim 8, in which said platform of the till device is secured on the upper end of a post which is adjustably connected with respect to height to the lower ram shaft.

10. Hydraulic metal powder press according to claim 8, including a stop element secured to the lower ram shaft, an operating element for interrupting the pressure medium supply to the lower ram, which operating element can be adjusted by the stop element, and including adjustable stops which are arranged spaced apart by the operating element, of which stops one lies above and the other one below the stop element being connected to the lower ram shaft, whereby by abutting of the stop element against the one or the other stop the operating element is moved in the one or in the other direction and through this the pressure medium supply to the lower ram is interrupted.

11. Hydraulic metal powder press according to claim 10, in which the adjustable stops are connected to indicating elements through which the position of the stops is visible.

12. Hydraulic metal powder press according to claim 11, in which each stop has a scale associated therewith.

13. Hydraulic metal powder press according to claim 12, in which each stop and the associate scale can be adjusted by common operating means.

14. Hydraulic metal powder press with a frame, a matrix with a mold cavity therein, which matrix can be moved in the frame in vertical direction through a lower ram, an upper ram with an upper press member which can be moved from above into the mold cavity of the matrix, which upper ram can be moved vertically in the frame, a lower press member which extends from below into the mold cavity, wherein the improvement comprises at least two stationary support posts which are adjustable in their length and the upper ends of each of which are free and serve as stops for the upper ram and limit its downward movement, said support posts consisting of a lower threaded spindle and an upper tube screwed onto the threaded spindle, which tube has at its upper end a pressure plate, and in which drive means for rotating the threaded spindle relative to the tube are provided, said drive means including one shaft common for the support posts, on which shaft worms are keyed and which engage worm gears arranged on the lower end of each threaded spindle. 

1. A metal powder press, comprising: frame means; lower ram means supported on said frame means for vertical movement relative thereto; a tool matrix mounted on said lower ram means and movable vertically therewith, said tool matrix having means defining an opening therethrough the axis of which is vertically aligned; lower press means receivable in the lower end of said opening in said tool matrix; upper ram means having upper press means thereon adapted to be moved into and out of the upper end of said tool matrix; metal powder storage means; and filling means connected to said metal powder storage means for selectively filling said opening in said tool matrix with a metal powder, said filling means comprising a platform and mounting means for mounting said platform in said lower ram means and being free of a direct attachment to said tool matrix but movable vertically with said tool matrix, a fill shoe and support means for supporting said fill shoe for sliding movement into and out of communication with said opening in said tool matrix and power drive means for moving said fill shoe between said in and out positions of communications with said opening.
 2. A metal powder press according to claim 1, wherein said filling means further includes means for vertically adjusting said platform relative to the upper surface of said tool matrix so that the plane of said platform can be adjusted to be flush and coplanar with said upper surface of said tool matrix.
 3. A metal powder press according to claim 2, wherein said adjusting means comprises a threaded support sleeve secured to said lower ram means and a threaded support column engaged with said threaded support sleeve and secured to said platform for supporting same, a hand wheel secured to said support column whereby a rotation of said hand wheel will rotate said support column to cause said threaded interconnection to effect an adjustment of said platform.
 4. A metal powder press according to claim 1, including a two part scale means, one part being connected in association with said frame means and the other part being connected in association with said tool matrix and including means for adjusting the relative positions therebetween.
 5. A metal powder press according to claim 1, including means for limiting the downward movement of said upper ram means.
 6. A metal powder press, comprising: frame means; lower ram means supported on said frame means for vertical movement; a hollow tool matrix supported on said lower ram means and movable vertically therewith; lower press means receivable in the lower end of said hollow tool matrix; upper ram means having upper press means thereon adapted to be moved into and out of the upper end of said tool matrix; and at least two adjustable stop means for limiting the downward movement of said upper ram means relative to said tool matrix, said stop means being in the form of supporting columns which are concealed within the press frame and finely adjustable in height by means of a common shaft with the aid of worm gears and having removable pressure plates at their upper ends.
 7. A metal powder press according to claim 6, wherein the supporting columns comprise a threaded portion which is driven by a worm gear, and a sleeve portion seated on the threaded portion, and the screw thread is covered with a protective sleeve.
 8. In a hydraulic metal powder press having a frame, a matrix having a mold cavity therein, which matrix is movable in the frame in vertical direction by a pressure-medium supplied lower ram shaft connected to same, an upper ram having an upper press member which can be moved from above into the mold cavity of the matrix, which upper ram is vertically movable in the frame and is supplied with pressure medium, a lower press member which extends from below into the mold cavity, a fill device for filling the mold cavity with metal powder, which fill device can be moved together with the matrix in vertical direction and which has a filling shoe which can be moved slidingly on the upper surface of the matrix and on a platform which lies in the same plane as the upper surface of the matrix, which filling shoe is connected to a storage reservoir for metal powder and includes a piston-cylinder arrangement for moving the filling shoe back and forth, comprising the improvement wherein said platform carrying the fill device is supported solely on said lower ram shaft and being free of any direct attachment to said matrix.
 9. Hydraulic metal powder press according to claim 8, in which said platform of the fill device is secured on the upper end of a post which is adjustably connected with respect to height to the lower ram shaft.
 10. Hydraulic metal powder press according to claim 8, including a stop element secured to the lower ram shaft, an operating element for interrupting the pressure medium supply to the lower ram, which operating element can be adjusted by the stop element, and including adjustable stops which are arranged spaced apart by the operating element, of which stops one lies above and the other one below the stop element being connected to the lower ram shaft, whereby by abutting of the stop element against the one or the other stop the operating element is moved in the one or in the other direction and through this the pressure medium supply to the lower ram is interrupted.
 11. Hydraulic metal powder press according to claim 10, in which the adjustable stops are connected to indicating elements through which the position of the stops is visible.
 12. Hydraulic metal powder press according to claim 11, in which each stop has a scale associated therewith.
 13. Hydraulic metal powder press according to claim 12, in which each stop and the associate scale can be adjusted by common operating means.
 14. Hydraulic metal powder press with a frame, a matrix with a mold cavity therein, which matrix can be moved in the frAme in vertical direction through a lower ram, an upper ram with an upper press member which can be moved from above into the mold cavity of the matrix, which upper ram can be moved vertically in the frame, a lower press member which extends from below into the mold cavity, wherein the improvement comprises at least two stationary support posts which are adjustable in their length and the upper ends of each of which are free and serve as stops for the upper ram and limit its downward movement, said support posts consisting of a lower threaded spindle and an upper tube screwed onto the threaded spindle, which tube has at its upper end a pressure plate, and in which drive means for rotating the threaded spindle relative to the tube are provided, said drive means including one shaft common for the support posts, on which shaft worms are keyed and which engage worm gears arranged on the lower end of each threaded spindle. 